Method for inerting activated carbon in biogas purification equipment

ABSTRACT

Various embodiments of the present invention are directed to methods of preparing a biogas purification system. Embodiments include flushing an adsorber with a gas stream that is separable by the downstream biogas purification process. Other embodiments include using the separable gas stream to flush a saturated adsorber. Additional embodiments include using a gas stream comprised of CO2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional No. 62/169,090entitled “Method for Inerting Activated Carbon in Biogas PurificationEquipment,” filed Jun. 1, 2015, the contents of which are herebyincorporated by reference in their entirety.

GOVERNMENT INTERESTS

Not applicable

PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not applicable

BACKGROUND

Not applicable

SUMMARY OF THE INVENTION

Various embodiments of the present invention are directed to methods ofpreparing a biogas purification system. Embodiments include flushing anadsorber with a gas stream that is separable by the downstream biogaspurification process. Other embodiments include using the separable gasstream to flush a saturated adsorber. Additional embodiments includeusing a gas stream comprised of CO₂.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example configuration of the present invention. Inthis configuration, biogas is admitted to adsorber A for the removal oforganics and H₂S. The valve to adsorber B remains closed. The effluentstream from adsorber A is passed to a separator for separation ofmethane and CO₂ such as a pressure swing adsorption (PSA). Once it is ofsufficient purity, the biomethane can then be admitted to the grid. Whenthe new adsorber, adsorber B, is installed, the CO₂ stream can bediverted from the vent to purge adsorber B until all of the air isremoved.

FIG. 2 illustrates an example configuration of the present invention. Inthis configuration, the valves to adsorber A, which now contains spentcarbon, are closed; the biogas stream is diverted to adsorber B, theoff-gases from which now pass to a separator such as a PSA unit. The CO₂separated therefrom is passed into adsorber A, and the methane andvolatile organics thereby displaced are flared until the flare isnaturally extinguished by the entrainment of the CO₂ purge gas. Oncethis is complete, the contents of adsorber A are rendered non-flammable,and the adsorber is ready for removal and transportation, followed byreplacement with a new adsorber B.

FIG. 3 illustrates oxygen concentration during CO₂ purging at a gasvelocity of 6.3 cm/second through a 250 mL test-rig.

FIG. 4 illustrates oxygen concentration during variable velocities ofCO₂ purging AP3-60 carbon,

FIG. 5 illustrates methane concentration during CO₂ purging at a gasvelocity of 3.2 cm/second through a 250 mL test-rig.

DETAILED DESCRIPTION

This disclosure is not limited to the particular systems, devices andmethods described, as these may vary. The terminology used in thedescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope.

The following terms shall have, for the purposes of this application,the respective meanings set forth below. Unless otherwise defined, alltechnical and scientific terms used herein have the same meanings ascommonly understood by one of ordinary skill in the art. Nothing in thisdisclosure is to be construed as an admission that the embodimentsdescribed in this disclosure are not entitled to antedate suchdisclosure by virtue of prior invention.

As used in this document, the singular forms “a,” “an,” and “the”include plural references unless the context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

“Biogas” refers to a renewable, environmentally friendly fuel thatcontributes less to global warming than do traditional fossil fuels.Biogas is produced by the aerobic fermentation of biomass from severalsources, including, but not limited to, domestic landfill, manure, rawsewage, sludges, and municipal solid waste.

Raw biogas is comprised mainly of a mixture of methane and carbondioxide gases, and may have small amounts of nitrogen, hydrogen sulfide,moisture, and siloxanes. The amounts of methane and carbon dioxidecomponents within biogas are variable, and are somewhat dependent uponthe organic matter precursor. CH₄ concentrations of 50-70 vol. % (mol.%) and CO₂ concentrations from 25-38 vol. % (mol. %) are consideredtypical. When it is purified to fossil natural gas standards, biogas isreferred to as “biomethane.” For this purification process, carbondioxide, hydrogen sulfide, water, and other organic contaminants must beremoved using one or more methods of separation

When the adsorbent media in a biogas purification plant is changed, alarge amount of air may be added to the biogas during start-up. Thedownstream process is not capable of handling a large amount of N₂ andO₂, the addition of which could downgrade the quality of the resultantbiomethane. Consequently, a method is required to reduce theconcentration of air in the biogas stream introduced to a separator suchas a PSA. Such methods and systems are particularly important when usingmobile filters, since the mobile filter is filled remotely from thecustomer site, where the filter media can then be flushed to remove airthat would otherwise be considered a contaminant.

Embodiments of the invention are directed to methods for purifyingbiogas and purifications systems configured for performing such methods.The methods of such embodiments may include the step of flushing asorbent with a flushing gas before contacting the sorbent with biogas.Although embodiments are not limited to particular flushing gas, incertain embodiments, the flushing gas may be separable from gassescollected during the purification process by subsequent downstreamprocessing. For example, in some embodiments, the gas may be CO₂. Thegas used to flush the sorbent may be obtained from an external sourceor, in particular embodiments, the gas may be obtained by recyclinggases from the purification process and using the recycled gas to flushthe sorbent. Other embodiments include methods that include the step offlushing spent saturated, or spent, sorbent to purge volatile organiccontaminants bound to the sorbent during the purification process. Onceflushed, the sorbent is suitable for carbon acceptance and compliantwith transport regulations regarding flammable material.

FIGS. 1 and 2 illustrate an example of a system used to implement themethods described above in which the flushing gas is recycled from adownstream portion of the system that includes two adsorber unitscontaining adsorbent: Adsorber A and Adsorber B. In FIG. 1, biogasenters the system and travels through valve 1 into Adsorber A, whereorganics and H₂S are removed. The effluent is transported through valve3 to a separator such as a pressure swing adsorption (PSA) unit wherebiomethane is separated from carbon dioxide (CO₂). The biomethane can befurther purified and admitted to the grid. The CO₂ removed is typicallyremoved from the system through vent valve 4. In embodiments of theinvention, the CO₂ can be redirected back through the system to flushAdsorber B, which may contain fresh sorbent, before introducing biogasinto Adsorber B. As illustrated in FIG. 1 in some embodiments, CO₂separated at the separator may be routed through valve 5 into Adsorber Band can be purged from the system through purge valve 6. Using twoadsorbent vessels (Adsorber A and Adsorber B) allows near-continuousproduction of the desired purified gaseous stream. It also permitspressure equalization, where the gas leaving the vessel beingdepressurized is used to partially pressurize the second vessel.

FIG. 2 illustrates the system of FIG. 1 arranged for purification ofbiogas through Adsorber B. Once the sorbent in Adsorber A is saturatedand Adsorber B has been flushed, the biogas stream can be diverted intoAdsorber B for removal of organics and H₂S by closing valve 1 andopening valve 9. Organics and H₂S can be removed from the biogas inAdsorber B and the effluent can be passed through valve 7 and into theseparator unit where biomethane is separated from CO₂. The CO₂ can berouted away from Adsorber B and through Adsorber A by closing valve 5and opening valve 8. The CO₂ can be used to purge the volatile organicsfrom Adsorber A through purge valve 2, where the volatile organics canbe burned off After purging, valve 8 can be closed and vent valve 4 canbe opened, so the sorbent can be removed from Adsorber A and disposedof. Fresh sorbent can be introduced into Adsorber A, and this sorbentcan be flushed by closing vent valve 4 and opening valve 8. The systemdescribed in FIGS. 1 and 2, therefore, provides for both flushing andpurging of the sorbent, while allowing for continuous flow of biogasthrough the system.

The sorbent used in Adsorber A and B can be any type of adsorbent knownin the art including, but not limited to, carbonaceous char, activatedcarbon, reactivated carbon, carbon black, graphite, zeolite, silica,silica gel, alumina clay, metal oxides, or a combination thereof. Insome embodiments, the sorbent can be a catalyst, or the sorbent can beimpregnated with one or more additives that aid in the adsorption oforganic impurities and/or hydrogen sulfide.

The catalytic sorbent material may be any sorbent material that cancatalyze a reaction known in the art. For example, carbonaceous char isknown to enhance a variety of oxidation reactions including, forexample, the oxidation of hydrogen sulfide (H₂S) and SO₂. Suchcarbonaceous chars act as true catalysts in this capacity because onlythe rate of a given reaction is affected, and the carbonaceous charsthemselves are not changed by the reaction to any significant degree.Thus, in some embodiments, the catalytic adsorbent material may be acarbonaceous char. In other embodiments, the catalytic adsorbentmaterial may be a carbonaceous char that has undergone processing toenhance catalytic activity.

In certain embodiments, the carbonaceous char may be prepared fromnitrogen-rich starting materials. Carbonaceous chars prepared fromnitrogen-rich starting materials have been shown to catalyze reactionssuch as hydrogen peroxide decomposition. In other embodiments, thecatalytic properties of a carbonaceous char that do not exhibitcatalytic activity or that have weak catalytic activity can be enhancedby exposing such chars to nitrogen-containing compounds such as urea,ammonia, polyamide, or polyacrylonitrile. In some embodiments, theexposure of the carbonaceous char to a nitrogen containing compound maybe carried out at high temperature such as greater than 700° C. and thecarbonaceous char may be heated before, during, or both before andduring exposure to the nitrogen containing compound. In otherembodiments, exposing the carbonaceous char to a nitrogen containingcompound may be carried out at less than 700° C., or low temperature. Instill other embodiments, the carbonaceous char may be oxidized at hightemperature before being exposed to a nitrogen containing compound.

The carbonaceous char or catalytic activated carbon may be calcined.Calcination may be carried out between any steps in the process. Forexample, in some embodiments, oxidized carbonaceous char may be calcinedbefore being exposed to a nitrogen containing compound, and in otherembodiments, the oxidized carbonaceous char may be calcined afterexposure to a nitrogen containing compound or after activation. In stillother embodiments, the carbonaceous char may be calcined between morethan one step in the process. For example, the carbonaceous char may becalcined after exposure to a nitrogen containing compound and afteractivation. Calcination is, generally, carried out by heating thecarbonaceous char or catalytic activated carbon to a temperaturesufficient to reduce the presence of surface oxides on the carbonaceouschar. The temperature at which surface oxides are removed may be fromabout 400° C. to about 1000° C. or any temperature there between, and insome embodiments, the calcination may be carried out in an oxygen-freeor otherwise inert environment.

Although embodiments of the invention include any means for separatingbiomethane from CO₂, in some embodiments such as those described in FIG.1 and FIG. 2, the means for separating biomethane from CO₂ may be apressure swing adsorption (PSA) unit. A PSA unit will typically includea plurality of vessels, each containing a bed of adsorbent material thatadsorbs a different gas such as water vapor, CO₂, N₂, and O₂. In someembodiments, the PSA will include vessels containing at least twodifferent adsorbent materials, and in certain embodiments, at least oneadsorbent material will be selective for methane, and at least oneadsorbent material will be selective for hydrogen. In some embodiments,the adsorbent materials may be positioned in the adsorption vessel inlayers. For example, the vessel may include one or more adsorbentmaterial layers selective for methane interspersed with one or moreadsorbent material layers selective for hydrogen. Each of the adsorbentmaterials may be activated carbon, carbon molecular sieves, zeolite,silica gel, alumina clays, and the like and combinations thereof.

The gaseous stream may be passed through the PSA under pressure. Thehigher the pressure, the more targeted gas component will likely beadsorbed, and when the pressure is reduced, the adsorbed gaseouscomponents can be released, or desorbed. Different target gasses tend toadsorb at different pressures. Therefore, PSA processes can be used toseparate one or more gas species from a mixture of gas species byadsorbing and releasing each species at a different pressure. AlthoughPSA is typically operated at near-ambient temperatures, heat can be usedto enhance the desorption of adsorbed species. For example, when airpassed under pressure through a vessel containing a bed of an adsorbentmaterial that is selective for nitrogen at a pressure that favorsnitrogen adsorption, substantially all of the nitrogen will adsorb ontothe adsorbent material, and the gaseous stream exiting the vessel willbe enriched in oxygen and depleted in nitrogen. When the bed reaches theend of its capacity to adsorb nitrogen, it is regenerated by reducingthe pressure, by applying heat, or both releasing the adsorbed nitrogen.It is then ready for another cycle of producing an oxygen enrichedstream.

In some embodiments, the biogas processing system may further include abiogas compression system, hydrogen sulfide cleaning system, a moistureknockout vessel, one or more compressors, biogas scrubber system, watersupply system, and analysis and final processing system. The variouscomponents of the systems of such embodiments may be connected by anysuitable means, such as piping, hoses, conduits, and the like andcombinations thereof. Such connecting means may conduct the materialhandled by the particular component of the system between components atthe required temperatures and pressures for proper operation. Thus,pressure and temperature may be maintained between components travelingthrough the connection means. Such systems may further include one ormore valves to control the flow of biogas between the components of thesystem.

In some embodiments, the system may include one or more digesters thatremove hydrogen sulfide from the biogas. Such digesters may bepositioned upstream or downstream of Adsorbers A and B as illustrated inFIG. 1 and FIG. 2 and may act in concert with Adsorbers A and B toremove hydrogen sulfide. In certain embodiments, digesters may bepositioned upstream of adsorbers A and B. Hydrogen sulfide degradesmetal equipment and sensors and is therefore typically removed early inthe processes and methods described herein. Additionally, hydrogensulfide is toxic, even when present at very minor concentrations,requiring its removal from the biogas stream.

In particular embodiments, the system may include a moisture separatorthat reduces the moisture content of the biogas. The moisture separatormay be upstream or downstream of Adsorbers A and B, and in certainembodiments, the moisture separator may be upstream of Adsorbers A and Band downstream of the digesters. In some embodiments, the moistureseparator may be positioned to remove moisture content present whenexiting the digester, and reduce the moisture content to less than about1.4%. Condensed moisture downstream may create problems for systemcontrol, as it interferes with gas flow and pressure measurements. Ifnot removed, condensation can also cause failure of compressor lube oilfilters and internal lubricated parts.

The systems of various embodiments may include one or more compressorsthat pressurize the biogas as it is transported through the system.Compressors may be incorporated at nearly any position in the system.For example, in certain embodiments, a compressor may be incorporatedinto the system near the start of the system, for example, upstream ofthe digesters and moisture separator, but downstream of Adsorbers A andB. In other embodiments, compressors may be positioned after Adsorbers Aand B, but before a PSA. In still other embodiments, compressors may bepositioned both upstream of Adsorbers A and B and downstream ofAdsorbers A and B. In further embodiments, a compressor may bepositioned after the PSA to pump bio-methane to a storage container, adevice configured to operate on bio-methane, or combinations thereof.The compressors of various embodiments may be powered by an electricmotor, a biogas-operable motor, or a crude methane-operable motor andthe like, and in some embodiments, generators may provide theelectricity necessary to power the compressors. Such generators mayinclude biogas-operable motor, or a crude methane-operable motor, andthe like. In particular embodiments, the motors may be powered by biogasor methane to allow the systems of embodiments to be self-contained.

In some embodiments, the systems may include an accumulator thatcombines the biogas streams with recycled gas streams coming from otherportions of the system such as a flash tank or gas drier. The combinedbiogas stream can then be directed from the accumulator into a systemupstream of, for example, Adsorbers A and B or the PSA. In otherembodiments, the combined biogas may be introduced into the systemupstream of the moisture separator.

In certain embodiments, the system may include a cooler which may reducethe temperature of the biogas stream below about 70° F. (about 21° C.).The cooler may be positioned anywhere within the system, and in certainembodiments, the cooler may be positioned immediately upstream of thePSA or a scrubber, which is described below.

Some embodiments of the systems of the invention may include a biogasscrubber. Such scrubbers typically remove carbon dioxide from biogasthrough water absorption. Carbon dioxide is more soluble in water underpressure than at atmospheric pressure, whereas methane is mostlyinsoluble in water, even at elevated pressures. Pressurizing amethane/carbon dioxide biogas mixture in the presence of water drivesthe carbon dioxide into solution in the water, but little methane isdissolved into solution. The gas flows in counter-flow or cross-flowwith the water. The resulting processed biogas has an enriched methanecontent, because some or all of the carbon dioxide has been processedout of the gas and into the water solution. The compressed operatingpressure is a function of the temperature, carbon dioxide mole fractionin the gas, and the desired methane purity. Such scrubbers may belocated downstream of a cooler, and are typically downstream ofAdsorbers A and B and downstream of the PSA. In certain embodiments, thescrubber system may be connected to a water supply system that pumpswater into the scrubber system.

EXAMPLES

Although the present invention has been described in considerable detailwith reference to certain preferred embodiments thereof, other versionsare possible. Therefore, the spirit and scope of the appended claimsshould not be limited to the description and the preferred versionscontained within this specification. Various aspects of the presentinvention will be illustrated with reference to the followingnon-limiting examples. The following examples are for illustrativepurposes only, and are not to be construed as limiting the invention inany manner.

Example 1

The dynamics of oxygen displacement by carbon dioxide flushing throughan activated carbon-filled adsorber were studied for engineering designpurposes. The number of bed-volumes required to inert a carbon filledcolumn was determined as a function of CO₂ gas velocity. Similarly, theinerting of a methane-loaded activated carbon column by CO₂ flushing hasbeen studied to determine the degree of purging required for compliancewith transportation and safety regulations.

In particular, the laboratory studies were required to provideexperimental data for evaluating the effectiveness of a CO₂ gas flushingprocedure to effect a safe ‘inerting’ of the activated carbon in servicefilter equipment used for biogas purification. Biogas is comprisedmainly of a mixture of methane and carbon dioxide gases, and may havesmall amounts of hydrogen sulfide, moisture, and siloxanes. The amountsof methane and carbon dioxide components within biogas are variable, andare somewhat dependent upon the organic matter precursor. CH₄concentrations of 50-70 vol. % (mol. %) and CO₂ concentrations from25-38 vol. % are considered typical.

The laboratory-scale ‘inerting’ tests were carried out at ambientconditions of temperature and pressure. Tests simulated flushing of abiogas adsorption system (when filled with an activated carbon) by 99.5%CO₂ gas. The laboratory ‘inerting’ studies simulated two separate,different stages of the filter equipment utilization, i.e., (a) oxygenremoval from a ‘new-filled’ adsorption system prior to its ‘on-stream’application for biogas purification; and (b) methane removal from a‘used’ carbon filter system, to thus ‘make-safe’ for transportation andde-commissioning operations (i.e., carbon emptying).

The studies used a vertical, cylindrical test-rig (250 mL internalvolume) and evaluated various rates of ‘down-flow CO₂’, thus providingdata for variations in contact time and gas velocity though the carbonbed. Testing evaluated separate flushing out with CO₂ gas of fourselected pelletized Chemviron Carbon grades. For comparison purposes,the extruded test carbons comprised two impregnated grades that aregenerally advocated for biogas purification, and two grades of basecarbon pellets (i.e., without impregnation) that represented theprecursor pellets for the two impregnated test carbons.

Test carbons included: (i) SOLCARB®KS3, a high performance impregnated 3mm pelletized product specifically developed for rapid vapor phaseremoval of hydrogen sulfide, organic mercaptans, and certain organicsulfides; (ii) ENVIROCARB® STIX®, an impregnated extruded carbon (4 mmpellets) designed for the removal of hydrogen sulfide, acid gases andother odorous compounds from air; (iii) ENVIROCARB® AP3-60, an extrudedbase carbon (3 mm pellets), which represented the precursor base forSOLCARB®KS3 manufacture; and (iv) ENVIROCARB® AP4-50, an extruded basecarbon (4 mm pellets), which represented the precursor carbon base forENVIROCARB® STIX® manufacture.

A rigid plastic tube test-rig was vertically oriented. Tube dimensionsare shown below in TABLE 1.

TABLE 1 Test-rig tube length 47 cm Internal diameter 2.6 cm Calculatedcross-sectional area 5.31 cm² Calculated internal volume 249.6 cm³

CO₂ gas flow (99.5% purity) was obtained from a regulated high-pressureCO₂ cylinder. CO₂ flow-rate control and measurement was conductedthrough a rotameter-type flow meter recalibrated for CO₂ gas.

The 250 ml test-rig was first filled with a recently obtained sample ofpelletized test carbon (no pre-drying), and the weight of test samplewas recorded. Under this carbon-filled condition, the test-rig alsocontained a certain proportion of air/oxygen; present in both theinter-pellet spaces, and also adsorbed in the carbon pore structure. Theaim of the inerting was to affect a removal of oxygen from the adsorbentcolumn system to <0.1% by ‘flushing out’ with a controlled downward flowof 99.5% purity CO₂ gas, at ambient conditions of temperature andpressure.

The initial trial used a sample of SOLCARB®KS3 pellets. The weight ofsample carbon used to fill the 250 mL test-rig was recorded. A CO₂flow-rate of 2 liters per minute was connected to the top of the carbonfilled test-rig column, and a digital timer was started. The 2liters/minute CO₂ gas flow corresponded to a calculated gas velocity of6.3 cm per second through the test-rig. At intervals during the courseof CO₂ gas flow and flushing-out, the effluent gas from the bottom ofthe adsorber was sampled by calibrated syringe, and was analyzed for itsoxygen component using the laboratory GC/MS (Agilent 7890A GasChromatograph and 5975C Mass Spectrometer) with the appropriate GCcolumn, pre-calibrated for O₂ concentration.

Due to the 2-minute oxygen residence time in the GC column, sampling ofeffluent gas from the test column was possible every 2 to 3 minutes. CO₂gas flow was continued through the test adsorbent until the O₂concentration was <0.1%. On completion of the adsorbent test, theflushed out carbon sample was removed from the test-rig, and its‘flushed out’ weight was recorded. Samples of the other three testcarbons were then similarly tested at a 6.3 cm/second velocity flow ofCO₂ gas, and the oxygen % results were recorded at several flow times. Asummary of the oxygen % test values for the flushing out at 2liters/minute flow CO₂ for the four carbons is shown in TABLE 2 below,and in FIG. 3.

TABLE 2 Four test carbons - O₂ concentration during CO₂ purging at gasvelocity 6.3 cm/second through 250 mL test-rig CO₂ flow rate 2 litersper minute CO₂ flow rate 8 empty bed-volumes per minute CO₂ velocitythough 6.3 cm per second carbon Solcarb KS3 STIX 4 mm AP3-60 base AP4-50base Bed- % Bed- % Bed- % Bed- % Volumes Oxygen Volumes Oxygen VolumesOxygen Volumes Oxygen  0 20.93 0 20.93 0 20.93 0 20.93  8 15.91 8 0.35 818.42 8 18.00 16 0.13 16 0.12 16 0.94 24 0.46 40 0.13 32 0.07 32 0.10 480.12 80 0.09 48 0.13 48 0.10 80 0.08 — — 80 0.12 80 0.10 — —

Test values at a CO₂ velocity of 6.3 cm/second indicated fairly rapidpurging of the oxygen from the test rig. For all the test carbons, the %O₂ in the effluent was reduced to <0.1% by about 30 to 40 bed-volumesflow (i.e., representing 4 to 5 minutes CO₂ flow). The two base carbonsappeared to take a slightly longer time for oxygen flushing than the twoimpregnated grades. For all test samples, there was no discernible‘exotherm’ during the CO₂ purging.

After scrutiny of the test values obtained with a CO₂ purge velocity of6.3 cm per second, additional purge gas velocities were similarlyassessed for the effect on the oxygen reduction. Purge flow rates of0.5, 1.0 and 6.0 liters CO₂ per minute were evaluated.

The ENVIROCARB® AP3-60 base carbon was chosen as the adsorbent for thevariable velocity purge testing, and was thought to represent thetypical purging properties of the other test carbons. After each purgetest, the ENVIROCARB® AP3-60 carbon was discarded and the test-rigrefilled with a ‘fresh’ amount of base carbon. The variable CO₂ purgegas conditions tested for the ENVIROCARB® AP3-60 carbon tested are shownin TABLE 3 below.

TABLE 3 Variable Purge Gas conditions for ENVIROCARB ® AP3-60 testcarbon CO₂ purging CO₂ purging CO₂ purging flow rate flow rate VelocityLiters per minute Bed-Volumes per minute cm per second 0.5 2 1.6 1.0 43.2 2.0 8 6.3 6.0 24 18.8

The % O₂ test values obtained in the gaseous effluent during thedifferent purge gas velocity conditions are shown in TABLE 4 and FIG. 4.

TABLE 4 AP3-60 carbon - Oxygen removal as a function of variable purgegas conditions Oxygen purging of AP3-60 base carbon CO₂ Flow-Rate 0.51.0 2.0 6.0 Litres per minute Bed-volumes per minute 2 4 8 24 Velocitycm per second 1.6 3.2 6.3 18.8 Bed- Bed- Bed- Bed- Vols % O₂ Vols % O₂Vols % O₂ Vols % O₂  0 20.93 0 20.93 0 20.93 0 20.93  2 20.80 4 16.53 818.42 8 19.80  6 4.17 8 4.39 16 0.94 16 1.14 10 0.59 16  0.08 32 0.10 240.14 16 0.23 — — — — 48 0.13 20 0.22 — — — — — —

Test values indicated an inverse relationship between CO₂ purgingvelocity and the reduction rate in oxygen concentration. That is, theslower the purging CO₂ velocity, the quicker the speed of oxygenremoval.

Values indicate that the purging effectiveness was primarily dependentupon adsorption (and diffusion) kinetics within the carbon/air system.(i.e., adsorption of CO₂ gas and removal of adsorbed air removal fromthe carbon pore structure). The initial air removal, and the continuingremoval of desorbed air/oxygen from the pore structure into theinter-pellet voids, would be relatively quick.

Notwithstanding the effects of adsorption kinetics, all the CO₂ purgevelocities tested removed oxygen from the test-rig fairly quickly, andindicated that about 16 to 20 bed-volumes of purging was effective.

Example 2

The aim of this study was to assess the effectiveness of a CO₂ purgeflow for the safe removal of methane from a ‘used’ biogas carbon filtersystem, to thus ‘make-safe’ before any transportation orde-commissioning operations.

The laboratory 250 mL test rig described above was again employed,together with the two impregnated pelletized carbon grades and theirrespective base carbon precursors. Prior to a carbon being tested, itwas filled into the test-rig and then allowed to saturate andequilibrate in a 1 liter per minute flow of 99.9% methane gas for 30minutes.

The methane-saturated test carbon was then similarly flushed out with a1 liter per minute purge flow of CO₂ gas. At timed intervals, a sampleof the effluent gas was analyzed for % CH₄, using the Agilent GC/MSpre-calibrated for methane concentration.

A summary of the methane % test values for the flushing out at 1liter/minute flow CO₂ for the four carbons is shown in TABLE 5 and FIG.5.

TABLE 5 Four test carbons - CH₄ concentration during CO₂ purging at gasvelocity 6.3 cm/second through 250 mL test-rig CO₂ flow-rate 1 liter perminute CO₂ flow-rate 4 empty bed-volumes per minute CO₂ velocity thoughcarbon 3.2 cm per second Solcarb KS3 STIX 4 mm AP3-60 base AP4-50 baseCO₂ Bed- % CO₂ Bed- % CO₂ Bed- % CO₂ Bed- % Volumes Methane VolumesMethane Volumes Methane Volumes Oxygen 0 100 0 100 0 100 0 100 4 98.93 496.71 4 97.27 4 95.62 12  1.06 12 0.15 12 0.18 12 0.15 24  0.10 24 0.1520 0.13 24 0.14

Methane gas was readily flushed from all the test carbons by the CO₂purge gas flow. There was very little difference between the four carbongrades tested. Methane concentration in the effluent gas stream wascirca 0.1% by about 12 bed-volumes purging with 100% CO₂ gas flow. Noheat of adsorption was evident during the methane purging tests.

The examples above demonstrate that oxygen removal from thecarbon-filled adsorbent vessel, prior to its use for biogaspurification, using a 99.5% CO₂ purge gas flow was effective.

A slower velocity purge gas flow of CO₂ was more effective for oxygenremoval from the adsorbent system than was a relatively faster velocitygas purge, due to the kinetics of air desorption rate from the carbonadsorbent.

Methane removal from a ‘used’ biogas purification vessel (prior to safede-commissioning and emptying) by flushing with a flow of 99.5% CO₂ gaswas very rapid and uncomplicated, with no noticeable evolution of heat.

The invention claimed is:
 1. A method for purifying a raw biogas, themethod comprising: providing an adsorber unit containing a sorbentmaterial; contacting the sorbent material contained within the adsorberunit with the raw biogas to adsorb organic impurities and/or H₂S andthereby form an effluent and a saturated sorbent material, wherein theraw biogas comprises (i) 25-38 vol.% CO₂, (ii) methane, and (iii)organic impurities and/or H₂S; separating CO₂ from the effluent, theseparated CO₂ forming a flushing gas; flushing the saturated sorbentmaterial with the flushing gas to produce a flushed sorbent; contactingthe flushed sorbent with the raw biogas; and producing a purified biogasfrom the effluent.
 2. The process of claim 1, wherein the sorbentmaterial is selected from the group consisting of carbonaceous char,activated carbon, reactivated carbon, carbon black, graphite, zeolite,silica, silica gel, alumina clay, metal oxides, and a combinationthereof.
 3. The process of claim 1, wherein the sorbent material isselected from the group consisting of catalytic adsorbent material,oxidized catalytic adsorbent material, calcined catalytic adsorbentmaterial, oxidized calcined catalytic adsorbent material, andcombinations thereof.
 4. The method of claim 1, wherein the purifiedbiogas is biomethane.